Broadcast Address

Broadcast address refers to the ability to address a message that is broadcast to all stations or hosts on a network. Ethernet networks are shared-media networks in which computers transmit signals on a cable that all other computers attached to the cable can receive. Thus, all the computers are part of the same "broadcast domain."

 

Normally, one computer transmits frames to only one other computer on the network by placing the MAC address of the destination computer in the frame. This frame is then transmitted on the shared media. Even though other computers see this frame on the network, only the target recieves it. A broadcast message is addressed to all stations on the network. The destination address in a broadcast message consists of all 1s (0xFFFFFFFF). All stations automatically receive frames with this address. Normally, broadcast messages are sent for network management and diagnostic purposes.

On IP networks, the IP address 255.255.255.255 (in binary, all 1s) is the general broadcast address. You can't use this address to broadcast a message to every user on the Internet because routers block it, so all you end up doing is broadcasting it to all hosts on your own network. The broadcast address for a specific network includes all 1s in the host portion of the IP address. For example, on the class C network 192.168.1.0, the last byte indicates the host address (a 0 in this position doesn't refer to any host, but provides a way to refer to the entire network). The value 255 in this position fills it with all 1s, which indicates the network broadcast address, so packets sent to 192.168.1.255 are sent to all hosts on the network.

Applications that produce broadcast messages include ARP (used by hosts to locate IP addresses on a network), routing protocols like RIP, and network applications that "advertise" their services on the network.